The invention relates to an apparatus for the contactless measuring of movement in one to three dimensions of the jaw and of the condyle in temporo-mandibular joint, respectively, by means of measuring devices comprising optoelectronic signal transmitters and signal receivers, between which light signals are transmitted either directly or by reflectors, part of the measuring device being secured to the lower jaw of the patient.
Prior to the present invention, optoelectronic devices for measuring the movement of the jaw were known. One such device is disclosed in the German published patent application, Offenlegungsschrift DE-OS No. 28 25 204 (page 12, lines 9 through 14). The signal receivers (sensors) of the apparatus disclosed in this patent application are mounted on a plate arranged in a plane and comprise a plurality of phototransistors which are interconnected electrically. The light sources of the apparatus are light signal transmitters such as light-emitting diodes. This apparatus allows measuring in each dimension because the integral signal received by the phototransistors depends on the distance between the phototransistors and the light source. The drawback of this system is that the measuring signal can easily be invalidated by unwanted light, especially when the distances between the light signal transmitter and signal receiver are great, and that there is no linearity between the generated output signal and distance between signal transmitter and signal receiver. A linear output can be achieved by means of electronic correction circuits but only at great expense and even then only approximately. Moreover, the arrangement of the plates with many signal receivers in combination with the signal transmitters are relatively complicated. The large number of receivers must be arranged on an expanded area and must be connected in parallel so that the integral received signal is responsive only to the distance between transmitter surface and receiver surface and not also additionally responsive--due to marginal zone effects--to the transverse displacement of the signal transmitter (and reflector, respectively) relative to the receiver surface whereby the measured result responsive to the distance would be invalid. As condyle movements in the temporo-mandibular joint into different directions are possible within the range of about 2 cm, the receiving surfaces must be large and must contain a large number of equivalent receivers in order to achieve homogeneity of receiption sensitivity over the total area of motion and beyond. If independence on the transverse displacements over the total area of motion is to be achieved, receivers must be provided far outside the area of motion where the light intensity is still measurable. Otherwise, marginal distortions invalidate the measured result.
In FIG. 4 of the above-mentioned published German patent application, the measuring head opposite the plates carrying light signal transmitters and receivers is ball-shaped and has an overall reflecting surface so that rotations of the reflector do not affect the measured result. According to the patent application disclosure, many transmitters and receivers are distributed on a large surface and connected in parallel in order to make the output signal independent of the transverse displacement of the reflector relative to the measured surface. This measuring device is very complicated and expensive. If the technical problems are to be reduced reducing the number of signal transmitter and receiver units, a restricted measuring surface and a measurement signal distorted by marginal effects is the necessary result.
The principle that the output of a photocell is a measure of the distance between a light source and photocell is described in J. Pros. Dent. Vol. 17, February 1967, pp. 109-121. As described in this article, several photocells are arranged side by side and connected parallel (cf. FIGS. 3 and 4 on page 111 and page 113, and the corresponding text) so that the output signal is not changed when the light source moves on a line maintaining equal distance from the photocells.
In Dental Clinics of North America, 13, 1969, pp. 629 to 642, a measuring device is described wherein six incremental transducers are provided as units (pp. 631 and 632). In each incremental transducer, a grating is moved between light emitting diodes and photocells, the movement of the grating being effected mechanically. The mounting ends of each incremental transducer are mechanically connected to the two face bows secured to upper and lower jaw. Because of this connection and the mechanical guide of the grating, the movement of the upper and lower jaw is not free of forces. Accordingly, an undisturbed recording of the movement of the lower jaw, which is controlled by sensitive neuromuscular reflexes and which is important for the temporo-mandibular joint diagnostics (TMJ-diagnostics), is impossible. Moreover, the signal received from each transducer is not a direct measure for the diagnostically important movement of the condyle in the temporo-mandibular joint in a given dimension, because each transducer measures, at a position far away from the temporo-mandibular joint, only the distance between the mounting points of the transducer at the two face bows when the lower jaw moves. A movement at the location of the joint can be determined only by a complicated calculation because of the mechanics of the rigid body.
The problem underlying the invention was to develop an apparatus of the type described above which has a simple construction and wherein the requirements on the evaluation circuit are not too severe and which delivers output quantities which are a direct measure of the movement of the jaw and of the condyle in the temporo-mandibular joint, respectively.